In continuous ink jet printing, electrically conductive ink is supplied under pressure to an orifice device, which can be an orifice plate, that distributes the ink to a plurality of orifices, typically arranged in a linear array(s), forming ink jets. The ink discharges from the orifices in jets that break into droplet streams. Individual drops in the droplet streams are selectively charged in the region of the break off from the filaments, and charge drops are deflected from their normal trajectories. The deflected drops may be caught and re-circulated. The undeflected drops are allowed to proceed to a print medium.
Drops are charged by a charge structure having a plurality of drop charging electrodes along one edge, and a corresponding plurality of connecting leads along one surface. The edge of the charge structure having the drop charging electrodes is placed in close proximity to the break off point of the ink jet filaments, and voltage applied to the drop charging electrodes induce charges on the drops as they break off from the filaments. The close proximity of the drop charging electrodes to the ink jet filaments and to the droplet streams makes these drop charging electrodes susceptible to contact with the inkjet filaments and the droplet streams. If ink makes contact with the drop charging electrodes, it can produce an electrical shorting condition between the drop charging electrodes and other components. If voltage is applied to the drop charging electrodes when such a shorting condition exists, damage can occur to the drop charging electrodes and/or to the component to which the drop charging electrodes are shorted to.
To protect these ink jet components, it is desirable to employ short detection means to detect potentially damaging shorting conditions between the drop charging electrodes and other components. One highly effective means for detecting shorting conditions utilizes short detection electrodes placed just below the drop charging electrodes. Ink that would short drop charging electrodes to other components will then make contact with the short detection electrodes where the shorting condition can be readily detected.
The placement of the short detection electrodes just below the drop charging electrodes results in these electrodes being more closely aligned with the droplet streams than are the drop charging electrodes. As a result, on occasion the droplet streams can make transient, incidental contact with the short detection electrodes when a shorting condition to the drop charging electrodes doesn't exist. Detection of the contact to the short detection electrodes by the associated short detection circuitry results in the initiation of a short recovery process, spotting print until the short recovery process is completed. Placement of the short detection electrodes just below the drop charging electrodes, while desirable for readily detecting potentially damaging shorting conditions, has therefore been seen to produce false readings that unnecessarily stop printing.
Inkjet heads require an extremely close alignment between an array of droplets and a series of drop charging electrodes. These electrodes are mounted to plates with circuitry known as a short detection circuit under each electrode to sense for fluid that could come in contact with these high voltage devices. If fluid touches this circuit, power is shut off to these electrodes to avoid damaging the drop charging electrodes or the orifice plate. The close proximity of the short detection circuit is necessary, but on occasion, the short detection electrodes interferes with the droplets of ink causing a false reading that stops printing.
Sutera U.S. Pat. No. 4,667,207 relates to a drop-catching structure for use in a liquid jet printing apparatus. The Sutera reference does not teach electrodes on the beveled face. Other references in the art related to charge plate fabrications are exampled in Morris U.S. Pat. No. 5,512,117 and Howell U.S. Pat. No. 4,928,113. References in the art related to the short detect electrode below charging are exampled in EP Patent Number EP0771655 (in particular, FIG. 3) and Fagerquist U.S. Pat. No. 4,994,821. The prior art listed herein is hereby incorporated by reference.
A need has existed for moving the short detection circuit away from the drop stream.
These embodiments described allow the movement of the short detection circuit away from the drop stream.